[Placental transfer of low-molecular weight heparin]

Prophylaxis of thromboembolism in pregnancy with low molecular weight heparin (LMWH) offers several advantages over conventional heparin (UFH) including the once daily application and the reduction of heparin-related side effects like osteoporosis, thrombocytopenia and allergy. Until now the use of LMWH in obstetrics failed because of a possible placental passage. The aim of our randomised controlled study with a total of 59 pregnant women was to control the placental passage of UFH and LMWH at term. 35 pregnant women received either 5000 I.U. UFH or 1500 aPTT units LMWH (Mono-Embolex) subcutaneously two hours prior to the termination or no injection. Maternal and fetal blood samples were taken after delivery to assay heparin activity by the heptest coagulation assay and the S2222 chromogenic substrate method. After application of LMWH heptest coagulation assay rose significantly from 20.8 +/- 2.0 to 40.3 +/- 9.4 s (p = 0.001), after UFH just from 21.6 +/- 1.5 to 22.9 +/- 3.7 s (p = 0.09). In the fetal blood samples, however, neither UFH nor LMWH could be measured. The data demonstrates that neither UFH nor LMWH used in this study crosses the placenta two hours after application and encourages the use of LMWH for the prophylaxis of thromboembolism during pregnancy.